Method of and apparatus for determining a response characteristic

ABSTRACT

A method of determining a response characteristic of a microwave device includes coupling a microwave resonator to the device, loading the resonator with microwave radiation, monitoring the time decay of power from the resonator, and comparing the monitored time decay with a known characteristic of the time decay of radiation in the microwave resonator when loaded with radiation.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to a method of determining a responsecharacteristic of a microwave device, and to analogous apparatus. Moreparticularly, the invention relates to apparatus comprising microwaveresonators together with precision timing equipment and methods of usingthese for calibrating the amplitude response in microwave equipment andcomponents.

2. Related Art

The requirement exists for calibrating the amplitude response ofmeasurement equipment and components for use in the microwave region ofthe electromagnetic spectrum (i.e. preferably in the frequency range 1to 300 or even 1000 GHz). In such equipment and components the outputsignal may be attenuated or amplified from an input signal and the ratioof output to input (the gain or attenuation) needs to be accuratelyknown.

General methods of measuring attenuation are described by F. L. Warnerin "Microwave Attenuation Measurement" (Published by Peter Peregrinus,London 1977).

Precision microwave attenuation standards are traditionally made fromresistance ratios which are carefully constructed to be frequencyindependent. At the highest level of precision such ratios may becompared with the National Standard. A variety of methods are used forNational Standards for microwave attenuation and these are reviewed byH. Bayer et al. "Attenuation and Ratio--National Standards", Proc. IEEE42, page 46, 1986. The UK primary standard is currently based on awaveguide beyond cut-off calibrator (See: R. W. Yell, "Development of aHigh Precision Waveguide Beyond Cut-off Attenuator", CPEM Digest 1972 pp108-110). In such standards, the microwave frequency is oftendown-converted in a linear mixer to a lower intermediate frequency (IF)and measurements made at the IF with high precision mechanicalapparatus. Such apparatus is capable of calibration to high resolution,for example to 0.0002 dB in 100 dB, with an accuracy of 0.01 dB in 10dB, but is complex and costly to construct. A common feature of knowncalibration techniques is that they calibrate the steady-state responseof the device under test.

The invention described herein preferably uses high Q microwaveresonators, where Q is defined by:

    Q.sub.1 =f.sub.O /Δf

Where Q₁ is the loaded Q of the resonator, f_(O) is the resonantfrequency of the resonator and Δf is the bandwidth of the resonance atthe half power (3dB) points. `High` Q in the context of this inventionis Q>10⁴, 10⁵ or 10₆. As of 1995, Q's as high as 10₁₁ are achievablepractically. Typical resonators known in the art include dielectricresonators (See: D. G. Blair et al. "High Q Microwave Properties of aSapphire Ring Resonator", J. Phys. D: Applied Physics, Volume 15, Page1651, 1982) and superconducting resonators (See: V. B Braginskii et al."The Properties of Superconducting Resonators on Sapphire", IEEE Trans.on Magn. Volume 17, Page 955, 1981, and C. D. langham and J. C. Gallop,"High Stability Cryogenic Sapphire Dielectric Resonator", IEEE Trans.Instrum. and Meas., 42, Page 96, 1993).

Superconducting resonators fabricated from low temperaturesuperconducting (LTSC) materials can demonstrate Q in the range 10₆ to10₁₁ but such devices must operate at a low temperature, typically lessthan 4.2K (-269° C.), using liquid Helium as coolant.

Development of high temperature superconducting (HTSC) oxide materialshas allowed fabrication of high Q resonators (Q>10⁶) from thesematerials which can operate at 77K (-196° C.) using liquid Nitrogencoolant. Different designs of resonator are known (See: S. J Fiedziuskoet al. European Patent Application (EPA) 0496512A1, K. Higaki et al EPA0522515A1 and EPA 0516145, and Z-Y Shen, PCT Application PCT/US92/09635)using different arrangements to make best use of HTSC materialsproperties.

SUMMARY OF THE INVENTION

It is an aim of the present invention to overcome the problemsencountered with the prior art, and more particularly to use the knowndecay of radiation in a high Q microwave resonator together withprecision timing apparatus to measure the amplitude response ofmicrowave equipment or components under test.

According to the present invention, there is provided a method ofdetermining a response characteristic of a microwave device comprisingcoupling a microwave resonator to the device, loading the resonator withmicrowave radiation, monitoring the time decay of power from theresonator, and determining the response characteristic of the device independence on the monitored time decay.

As used herein, the term "microwave device" connotes broadly any type ofmicrowave equipment or component, such as microwave spectrum analysers,microwave amplifiers, microwave signal generators, or more simplecomponents such as resistors and the like. The response characteristicof a resistor would typically be the voltage drop across the resistor.

The resonator may be coupled to the device either before or after beingloaded with microwave radiation. If before, then both the resonator andthe device would be loaded with radiation prior to the monitoring step.It will be appreciated that this would not usually affect the test inquestion, since the capacity of the device under test to store microwaveenergy would invariably be negligible.

It will be understood from the foregoing that the present inventionpreferably provides a method of measuring or calibrating the amplituderesponse of microwave equipment or components utilising accurate timingmeans to monitor the time decay of power in a microwave resonator loadedor filled with microwave radiation when such radiation has passedthrough the equipment or component under test and comparing the observedtime decay with the known time decay of radiation in a microwaveresonator loaded with radiation.

The present invention also preferably provides a method of calibrating aresponse characteristic of a microwave device comprising providing amicrowave resonator coupled to the device, filling the resonator withmicrowave radiation, monitoring the time decay of power from theresonator in the device or as indicated in the device, and comparing themonitored time decay with the known time decay of radiation in themicrowave resonator, when filled with radiation but in the absence ofthe device.

A basic feature underlying the preferred embodiments of the invention isto use the known characteristic of the radiation in a microwave cavity,that the radiation decays exponentially, to plot deviations from idealof amplitude response in test equipment. This is done by comparing thetime decay of the radiation having passed through the test equipmentwith the accurately known exponential decay of radiation in a bareresonator. Hence, power, which is a difficult quantity to measuredirectly, is effectively evaluated in terms of time, which is aconsiderably easier quantity to measure.

The decay of the radiation is advantageously sufficiently long fortiming to sufficient accuracy to be made. As timers are available whichcan measure with an accuracy in the nanosecond regime, this implies useof a high Q resonator.

Preferably, the resonator is fabricated from superconducting materialand operated at a temperature below the superconducting transitiontemperature.

Preferably, the superconducting material has a transition temperatureabove 30 or 60K, more preferably above 70 or even 80K.

Preferably, the resonator is held at a temperature below thesuperconducting transition temperature using a closed cyclerefrigerator.

Preferably, the resonator comprises dielectric material. As used herein,the term "comprises" connotes, inter alia, "includes" or "contains".

Preferably, a microwave source is provided for loading the resonatorwith microwave radiation. Alternatively (or additionally), an amplifiermay be provided, coupled to the resonator, to amplify power in theresonator. The former case, using an "external" microwave source, can berelatively simple but relative expensive to implement in practice. Thelatter case, which may operate on the loop oscillator principle, caneffectively utilise the resonator itself as the microwave source. Thiscan be relatively cheap but relatively complicated to implement.

The present invention extends to apparatus for determining a responsecharacteristic of a microwave device, comprising a microwave resonator,means for coupling the resonator to the device, means for loading theresonator with microwave radiation, means for monitoring the time decayof power from the resonator, and means for determining the responsecharacteristic of the device in dependence on the monitored time decay.

Apparatus features analogous to the aforementioned method features arealso provided.

Hence, preferably, the resonator comprises superconducting material andthe apparatus is adapted to operate the resonator at a temperature belowthe superconducting transition temperature.

Preferably, the superconducting material has a transition temperatureabove 60K.

Preferably, the apparatus further comprises a closed cycle refrigeratorfor holding the resonator at a temperature below the superconductingtransition temperature.

Preferably, the resonator comprises dielectric material.

Preferably, the apparatus includes a microwave source for loading theresonator with microwave radiation.

Preferably, the apparatus includes an amplifier, coupled to theresonator, for amplifying power in the resonator.

With the present invention, preferably a resonator is first loaded upwith microwave electromagnetic energy (Frequency range 1 to 300 GHz)until a steady state is reached where the input power is equal to thesum of the power dissipated in and that radiated from the resonator. Theoutput port of the resonator is connected to the attenuator, amplifieror other device to be calibrated. When a steady state is achieved forpower in the resonator an accurate trigger and clock generator opens afast switch, cutting off the input power to the resonator. As a resultthe stored energy U(t) and the radiated power P(t) both decay in anaccurately exponential manner with time:

    P(t)∝U(t)=U(O)exp(-2πΔft)                  Equation 1

where Δf is the 3dB linewidth of the resonance at frequency f₀ and isrelated to the loaded Q₁ by the expression:

    Q.sub.1 =f.sub.O /Δf

The higher the Q the greater the accuracy of the microwave power scaleP(t) since, provided signal-to-noise considerations do not limitperformance, it is the timing accuracy δt of the instant t at which acalibration is made which sets the limiting resolution δP of the device.Thus:

    δP∝U(O) exp (-2πΔft)δt/2πΔf

Operation in the time domain is preferable to operation in the frequencydomain because time intervals can be readily measured to high precision.Thus use of a high Q resonator, together with a precision timer, gives adevice which can generate microwave output power whose amplitude decaysas an accurately exponential function with time. Amplitude ratios, andhence attenuation or amplification ratios can then be derived fromtiming interval ratios.

In practice, the power decay from the isolated resonator can be measuredin the piece of equipment under test as a function of time, using theexternal accurate timing circuit as a reference. Deviations from theexponential decay of resonator output power amplitude will then indicatenon-linearities in amplitude response or attenuation ratio in theequipment or component under test.

It will be understood from the foregoing that one known characteristicof the time decay of radiation in a microwave resonator, known withoutany experimental determination but purely from theoreticalconsiderations, is that the decay is exponential. Comparison of themonitored time decay with an exponential form can indicatenon-linearities in the response characteristics of the device undertest.

Another known characteristic could be the time constant of theexponential decay. This would usually need to be measuredexperimentally. Comparison of the monitored time decay with this timeconstant can yield information concerning the gain or attenuation of thedevice in question.

In a prototype apparatus, using a cavity resonator fabricated from leadcontaining sapphire dielectric, operating at 4.2K (-269° C.) a Q inexcess of 10⁷ can be achieved. In such a resonator, microwave poweramplitude output, if operated as described above, decays in a timescaleof 1 to 10 milliseconds. Timing accuracy can be achieved over such aperiod such that attenuation ratios of the order 0.01 dB can beresolved, say, in a 10, 20 or 30 dB range. This is adequate forcalibrating apparatus or devices and has the advantage of being a directmeasurement at the resonator frequency, without requiringdown-conversion or precision mechanical apparatus.

Whilst the prototype apparatus uses low temperature superconductor(LTSC) material, it is known that high Q resonators can be fabricated ofhigh temperature superconductor (HTSC) oxide material. Such resonatorscan demonstrate Q in excess of 10⁷ below the superconducting transitiontemperature of the material (90K or -183° C.) and can operate in liquidNitrogen coolant (Boiling point 77K or -196° C.).

Fiedziusko et al. (European Patent Application 0496512A1) have shownthat such a resonator can be made sufficiently small that operation ispossible with a closed cycle cooler (for example a Stirling cycle coolercapable of cooling to 60K or -213° C.) such as are commerciallyavailable. Use of such a cooler has the practical advantage that noliquid cryogen is then necessary to cool the resonator to below thetransition temperature of the superconductor, giving a HTSC resonatorhaving sufficiently high Q to be used in this invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred features of the present invention are now described by way ofexample with reference to the accompanying drawings, in which:

FIG. 1 is a schematic block diagram of one embodiment of this inventionarranged to measure the response of an attenuator, amplifier or detectorin a piece of equipment under test using an external microwave source toload the resonator;

FIG. 2 is a schematic block diagram of another embodiment of thisinvention arranged to measure the response of an attenuator, amplifieror detector in a piece of equipment under test using a loop oscillatorconfiguration;

FIG. 3 is a schematic block diagram of a further embodiment of thisinvention arranged to measure the response of a component under testusing a loop oscillator configuration;

FIG. 4 is a graph representing the signal decay with time from theisolated resonator, and the decay observed in the equipment or componentunder test;

FIG. 5 is a schematic block diagram of a further embodiment of thisinvention arranged to measure the response of an attenuator, amplifieror detector in a piece of equipment under test using a loop oscillatorconfiguration where the timing means has been replaced by a countercounting oscillation cycles in the radiation decaying in the resonator;and

FIG. 6 is a graph representing the signal decay as a function of thenumber of oscillation cycles of the radiation decaying in the isolatedresonator, and the decay observed in the equipment or component undertest.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

A first preferred embodiment of the invention is illustrated in FIG. 1where a resonator 9 is arranged inside a cooled enclosure 10. Examplesof resonators which may be used include cavity dielectric loadedresonators, stripline resonators and coaxial resonators. Coaxialresonators may be used where measurements at a number of frequencies maybe needed because such resonators may be tuned to a number of resonantmodes having equally spaced frequencies. The resonators may be formedfrom low temperature superconductor LTSC) material or high temperaturesuperconductor (HSC) material, with or without dielectric loading, butwould preferentially be formed of dielectric material in a HTSCenclosure. The cooled enclosure 10 may be a cryostat or refrigeratorcapable of operation below the transition temperature of the materialfrom which resonator 9 is fabricated.

A stable microwave source 1 is connected by a conductor, waveguide orsimilar means 2 to the input of directional coupler 5. Examples ofmicrowave source 1 could include signal generators and microwaveoscillators. The first output of directional coupler 5 is connected tofast switch 4. Fast switch 4 may, for example, be a PIN diode switch,but any switch capable of switching in less than the decay time formicrowave radiation in resonator 9 may be used. Fast switch 4 operatesunder the control of timer/controller 13. The output of fast switch 4passes to the input 7 of resonator 9. The second output of directionalcoupler 5 is connected to a counter 6.

The output 8 of the resonator 9 is connected via a conductor, waveguideor similar means 2, via an optional low noise amplifier 11 and anoptional variable attenuator 12, to the input of equipment under test14. Timer/controller 13 also provides accurate timing reference signalsto the equipment under test 14.

It will be appreciated that the lines shown in FIG. 1 from thetimer/controller 13 to the fast switch 4 and the equipment under test 14signify paths for communicating control and like signals.

Examples of equipment under test 14 would include microwave spectrumanalysers, microwave amplifiers, microwave signal generators and similarequipment. Such equipment usually provides an output proportional to theinput, the proportionality being set by the internal attenuator oramplifier gain setting. The proportionality relationship between inputand output in such equipment may be calibrated by the present invention.

Finally, the results of the calibration test are passed from the outputof the equipment 14, via the timer/controller 13, to a determining andcomparison means 40, for comparison with the known time decay ofradiation in the resonator 9. The determining and comparison means wouldtypically comprise a suitably programmed digital computer. If, as wouldperhaps be particularly common, it were desired to determine thenon-linearities in the gain or attenuation of the device under test bycomparison with the theoretically known exponential decay characteristicof the microwave resonator, this could be achieved by programming thecomputer to determine the logarithm of the monitored time decay,evaluate a linear least squares fit, and evaluate from the departurefrom the exponential behaviour the non-linearities of the device (henceeffecting a comparison with the truly exponential behaviour of theresonator itself). Such an evaluation would yield information concerningthe variation of gain (or amplitude response) at different power levels.

In operation, the resonator 9 is held at a temperature below thecritical temperature of the resonator material by cooled enclosure 10.Fast switch 4 is held closed under the control of timer/controller 13.The stable microwave source 1 generates power which passes viadirectional coupler 5 and fast switch 4 to the input 7 of the resonator9. The frequency of microwave source 1 is monitored by counter 6connected to the second output of directional coupler 5. Energy storedin the resonator 9 increases until power into the resonator via 7 ismatched by power loss in the resonator and power dissipated through theresonator output 8. Power from output 8 passes to an amplifier 11 andthen, via attenuator 12, to the equipment under test 14.

Low noise amplifier 11 and attenuator 12 are optional and may beomitted.

When a steady state of power in resonator 9 is achieved, fast switch 4is opened under the control of timer/controller 13. The decay ofmicrowave power in resonator 9, passing via amplifier 11 and attenuator12 to the equipment under test 14 is accurately timed by thetimer/controller 13. Deviations from the known exponential decay inpower in resonator 9 appearing at the output of the equipment. undertest 14 then indicate departure from linearity in the amplitude responseof 14.

It will be understood that, as explained above, dependent upon thespecific type of calibration to be performed, the actual time constantof the known exponential decay in power in the resonator need not bedetermined experimentally, although it could be if desired; usually itsuffices to know that the decay in power is exponential in form.

If amplifier 11 and attenuator 12 are included, a correction for anyknown non-linearity of these components may be applied.

FIG. 4 shows a representation of the signal decay with time. The uppercurve 23 represents the signal decay in the resonator after the fastswitch is opened and is known to accurately follow an exponential decayas defined in Equation 1. The time for the signal to pass from onepredetermined level 24 to another 25 is measured by the time interval 27to 29. The lower curve 22 represents the signal decay with time in theequipment under test, although in practice this curve would have a morecomplex form than that shown. The time to pass from level 24 to level 25in the equipment under test is represented by the interval 26 to 28. Thetime interval 27 to 29, which represents a known change in signal levelcan then be compared with the interval 26 to 28 to derive the departurefrom theoretical of the amplitude response of the equipment under test.

It will be understood that the time constants of the two curves shown inFIG. 4 would be virtually identical. Differences between the two curvesrepresent the non-linearities of the device under test. Suchnon-linearities would not usually be expected to exceed 10% or so.

Whilst only two signal levels arc shown in FIG. 4, in practice aplurality of signal levels would be chosen and timing intervals betweenthese for the equipment under test would be compared with the knowndecay times for the resonator to give attenuation or amplificationratios over a range of values.

It will be appreciated that the measurements described above are made ata single frequency set by the high Q resonator 9. If a multi-moderesonator, such as a coaxial resonator is chosen, then the operatingfrequency of the microwave source 1 can be adjusted to be at anotherresonant mode of the resonator, so that resonator 9 fills with radiationat a second frequency, monitored by counter 6. Timing measurements asdescribed above can be repeated at this second frequency and subsequentfrequencies at which the resonator has resonant modes. Alternatively, tooperate at different frequencies, resonator 9 may be tuned to operate ata different frequency or changed for another resonator which resonatesat a different frequency.

FIG. 2 illustrates an alternative embodiment, where the resonator isconnected as a loop oscillator. In this arrangement, the resonator 9 isheld at a temperature below the critical temperature of the materialfrom which 9 is fabricated by cooled enclosure 10. The resonator output8 is connected through a conductor, waveguide or similar means 2 tofirst directional coupler 15. One output of first directional coupler 15is connected to amplifier 11 whilst the second output of firstdirectional coupler 15 is connected to the equipment under test 14. Theoutput of amplifier 11 is connected via attenuator 12 to the fast switch4, which operates under the control of the timer/controller 13.Timer/controller 13 also supplies accurate timing reference signals tothe equipment under test 14. The output of switch 4 passes through phaseshifter 3, to a second directional coupler 5, the output of which isconnected to the input 7 of resonator 9. The second output of seconddirectional coupler 5 is connected to counter 6.

In this configuration, thermal excitation within resonator 9 at itsresonant frequency is amplified in amplifier 11. Suitable adjustment ofattenuator 12 and phase shifter 3 can make oscillation of resonator 9self sustaining at one of its resonant modes, so power inside resonator9 builds to a steady level, at a frequency monitored by counter 6.

The advantage of this configuration is that a stable microwave source isnot needed. Amplifier 11 can be fixed gain and its bandwidth is notcritical because the self excitation of high Q resonator 9 generates therequired narrow frequency band.

Alternatively, amplifier 11 could incorporate a band-pass filtercentered on the resonant frequency of the selected oscillation mode.

As described above, power in the resonator 9 is allowed to build to asteady level. Fast switch 4 is opened under control of timer/controller13, which also accurately times decay of the power from resonator 9through the equipment under test 14.

As also described above, and illustrated in FIG. 4, the timing ratios ofperiods for the signal to pass between predetermined levels, as measuredin equipment under test 14, can be compared with the known decay ofpower in the resonator 9 to give measurement of deviations in amplituderesponse in the equipment under test 14.

If operation of the loop oscillator configuration in FIG. 2 is requiredat different frequencies then a multi-mode resonator, such as a coaxialresonator, may be chosen for resonator 9. Amplifier 11 and attenuator 12must then be selected to have adequate gain over all of the resonantmode frequencies of resonator 9. As the loop oscillator configuration isself exciting using thermal oscillations in the resonator 9, modeselection is achieved by adjustment of phase shifter 3 to give a phaseshift of an integral of 2π at the resonator input 7, relative to itsoutput 8 at the selected resonant frequency of resonator 9. The newoperating frequency of the loop oscillator is monitored by counter 6.

FIG. 3 represents another embodiment, using a loop oscillatorconfiguration to measure the amplitude response of components undertest, or to compare components under test. As noted above, microwaveequipment usually has an output which is proportional to the input. Whentesting components, however, some means is preferably included toquantify the signal emerging from the component under test.

In the embodiment illustrated in FIG. 3, the resonator 9 is held at atemperature below the critical temperature of the material from whichthe resonator is constructed by cooled enclosure 10. The output 8 ofresonator 9 is connected by a conductor, waveguide or similar means 2 tofirst directional coupler 15. The first output of first directionalcoupler 15 is connected to amplifier 11, which in turn is connected viaattenuator 12 to first fast switch 4 which operates under the control oftimer/controller 13. The output of first fast switch 4 connects viaphase shifter 3 to second directional coupler 5, the first output ofwhich is connected to the input 7 of resonator 9 whilst the secondoutput of second directional coupler 5 is connected to counter 6.

As described above, with first fast switch 4 being held closed by timercontroller 13, the loop formed by components 9, 15, 11, 12, 4, 3 and 5forms a loop oscillator, where thermal oscillations in resonator 9 areamplified and fed back so that microwave power at the resonant frequencyof resonator 9 builds inside the resonator to a steady level.

The second output of first directional coupler 15 is connected to theinput of the component under test 16 and to the second input 18 ofsecond fast switch 19. The output of component under test 16 isconnected to the first input 17 of second fast switch 19. In practice,the connection between the second output of directional coupler 15 andthe second input 18 of second fast switch 19 would be chosen to be equalin length to the connection from second output of directional coupler 15through the component under test 16 to first input 17 of second fastswitch 19.

Second fast switch 19 operates as a change-over switch, so that eitherthe first input 17 or the second input 18 may be connected to the switchoutput under the control of timer/controller 13. The output of secondfast switch 19 passes through diode 20 to fast indicator 21. Indicator21 may be a fast analogue to digital converter (ADC) or similar meansfor recording the output from fast switch 19. The output may, forexample, be recorded as a d.c. voltage signal. Indicator 21 alsooperates under the control of timer/controller 13 which also suppliesaccurate timing reference signals.

In operation, first fast switch 4 is held closed by timer/controller 13.Thermal oscillations in resonator 9 are amplified and fed back intoinput 7 to form a self exciting oscillator at the resonant frequency ofresonator 9, so that power in resonator 9 builds to a steady value at afrequency monitored by counter 6. Timer/controller 13 then connects thefirst input 17 of second fast switch 19 to the output of second fastswitch 19, which connects via diode 20 to indicator 21 and the steadyreading on indicator 21 is recorded. This steady reading on indicator 21represents the signal level which has passed through device under test16. The timer/controller 13 then opens first fast switch 4, whilstswitching the second input 18 of second fast switch 19 into connectionwith the output. Power in resonator 9 then passes through output 8 tofirst directional coupler 15 and then from the second output of firstdirectional coupler 15 to the second input 18 of second fast switch 19where it passes to the output of second fast switch 19. The output ofsecond fast switch 19 passes through diode 20 and the output of 20 isrecorded by indicator 21. Accurate timing signals are passed fromtimer/controller 13 to indicator 21. In this way the decay in time ofpower in resonator 9 is recorded in indicator 21.

As described above, with reference to FIG. 4, timing intervals betweentwo or more preselected signal levels can be used to compare theamplitude response of the component under test with the known signaldecay in the resonator. In the apparatus in FIG. 3, as described above,the decay in time is observed without the component under test in thecircuit and the time delay recorded for the signal level to decay to thesteady state signal level present at 21 when connected to the deviceunder test 16. In this way a single decay measurement can determine theattenuation in the device under test 16 once the decay characteristic ofthe resonator has been determined. A similar arrangement may be used ifcomponent under test 16 is an active rather than passive component, inwhich case the component gain can be measured.

An alternative arrangement might use measurement of two decay responseswith time, using the well known series or parallel substitution methodsto compare components.

The technique for determining the amplitude response of the componentusing the determining and comparison means 40 is now described. Firstly,the exponential decay characteristic of the resonator is determined, inthe absence of the component under test. From this decay characteristicis known the rate of decay of power in the resonator in dB's per second.This information is passed to the comparison means 40.

Secondly, the determining and comparison means converts the time delayobserved without the component under test in the circuit to decay to thesteady state signal level present when connected to the device undertest, into a power value using the known rate of decay of power in theresonator.

It will be appreciated that the comparison which is effected is betweenthe monitored time decay and a predetermined rate of time decay for theresonator itself.

If measurements on components under test are required at differentfrequencies then a multi-mode resonator, such as a coaxial resonator,may be chosen for resonator 9. Amplifier 11 must then be selected tohave adequate gain over all of the resonant mode frequencies ofresonator 9. As the loop oscillator configuration is self exciting usingthermal oscillations in the resonator 9, mode selection is achieved byadjustment of phase shifter 3 to give a phase shift of an integral of 2πat the resonator input 7, relative to its output 8 at the selectedresonant frequency of resonator 9. Timing measurements as describedabove can be repeated at the second and subsequent frequencies.

In the apparatus as described with reference to FIG. 3, it will beunderstood that the means for quantifying the signal emerging from thecomponent under test comprises the fast switch 19, the diode 20, and theindicator 21.

A further embodiment of the invention is shown in FIG. 5. Thisembodiment differs from those previously described in that no externalprecision timer is required. This is possible because the high Q of theresonator implies a very narrow bandwidth for the radiation in theresonator at its resonant frequency. As the preferred form of theresonator is a dielectric loaded superconducting resonator operating ata temperature below the critical temperature of the superconductor, sucha resonator can form a very stable oscillator having a narrow bandwidth.Accordingly, instead of using external timing means, oscillation cyclesof radiation in the resonator can be counted, in a suitable counter, toform the timebase for the radiation decay. As the resonant frequency ofthe resonator is chosen to be >1 GHz, then each cycle of oscillation ofradiation in the resonator lasts <1 nanosecond.

As shown in FIG. 5, the apparatus is again configured in a looposcillator arrangement. In this arrangement, the resonator 9 is held ata temperature below the critical temperature of the material from whichresonator 9 is fabricated by cooled enclosure 10. The resonator output 8is connected through a conductor, waveguide or similar means 2 to firstdirectional coupler 15. One output of first directional coupler 15 isconnected to amplifier 11 whilst the second output of first directionalcoupler 15 is connected to the equipment under test 14. The output ofamplifier 11 is connected via attenuator 12, through second directionalcoupler 5, to the fast switch 4, which operates under the control of thecontroller 30. The second output of second directional coupler 5 isconnected to counter 6. Controller 30 also supplies control signals tocounter 6 and to the equipment under test 14. The output of switch 4passes through phase shifter 3 to the input 7 of resonator 9. In thisconfiguration, amplifier 11 does not require high linearity because onlythe number of oscillation cycles are to be counted.

As described above, fast switch 4 is held closed under the control ofcontroller 30 and thermal excitation within resonator 9 at its resonantfrequency is amplified in amplifier 11. Suitable adjustment ofattenuator 12 and phase shifter 3 can make oscillation of resonator 9self sustaining at one of its resonant modes, so power inside resonator9 builds to a steady level.

When power in the resonator has built to a steady level, controller 30opens fast switch 4 and simultaneously resets counter 6 which begins tocount oscillation cycles in the resonator. The cumulative count incounter 6 is used as the timebase to measure the signal decay recordedin the equipment under test 14.

FIG. 6 shows a representation of the signal decay against accumulatedoscillation cycles as measured by the counter 6 in FIG. 5. The uppercurve 32 represents the signal decay in the resonator after the fastswitch is opened and is known to accurately follow an exponential decayas defined in Equation 1. The interval for the signal to pass from onepredetermined level 33 to another 34 is measured by the number ofoscillation cycles between 36 and 38. The lower curve 31 represents thesignal decay in the equipment under test against accumulated oscillationcycles measured by the counter, although in practice this curve wouldhave a more complex form than that shown. The interval to pass fromlevel 33 to level 34 in the equipment under test is represented by thecount range 35 to 37. The number of oscillation cycles in the interval36 to 38 which represents a known change in signal level can then becompared with the number of oscillation cycles in interval 35 to 37 toderive the departure from theoretical of the amplitude response of theequipment under test.

Whilst the number of oscillation cycles may be measured from the timethat fast switch 4 in FIG. 5 is opened and the count difference betweenpoints 36 and 38 and 35 and 37 are taken, an alternative would be to usepreset signal level 33 to start the count and signal level 34 to stopthe count in the measurement of signal decay. This might be readilyachieved using signal comparator circuits, or similar means (such assignal amplitude circuits, based on a detector diode and a voltagecomparator circuit, which produce an output signal when the inputvoltage falls within a predetermined range) set to levels 33 and 34 inFIG. 6 which would then generate signals to start and stop the counter.Multiple comparators may be used if measurements are needed at a numberof signal levels or, alternatively, the form of the whole decay curves31 and 32 in FIG. 6 may be plotted against cumulative count.

It will be appreciated, in the light of the disclosures above, thatapparatus as described in FIG. 3 for the testing of components may beconfigured in a form similar to that described in FIG. 5 so thatoscillations of radiation in the resonator may be counted in a counteras an alternative to using a precision timer. Similarly, measurementsusing apparatus described in FIG. 5 may be made at a multiplicity offrequencies using different resonant modes of the resonator as describedabove.

In the preferred embodiments, all of the components of the relevantembodiment, including the cooling means for the cooled enclosure 10, arehoused within a single housing. In order to achieve this, the coolingmeans needs to be of a practicable size, and hence would usually be aclosed cycle cooler.

It will be understood that the present invention has been describedabove purely by way of example, and modifications of detail can be madewithin the scope of the invention.

Each feature disclosed in the description, and (where appropriate) theclaims and drawings may be provided independently or in any appropriatecombination.

What is claimed is:
 1. A method of determining a response characteristic of a microwave device, said method comprising the steps of:coupling a microwave resonator to said device, supplying said resonator with microwave electromagnetic energy until a steady state condition is reached in which input power equals the power dissipated in and radiated from said resonator; cutting off the supply of microwave electromagnetic energy to said resonator; monitoring the time decay of radiated power from said resonator; and determining the response characteristic of said device in dependence on said monitored time decay.
 2. A method as in claim 1 wherein said determining step comprises the step of comparing the monitored time decay with a known characteristic of said time decay of radiation in said microwave resonator when supplied with radiation.
 3. A method as in claim 1 wherein said resonator is fabricated from superconducting material and operated a temperature below the superconducting transition temperature.
 4. A method as in claim 3 wherein said superconducting material has a transition temperature above 60K.
 5. A method as in claim 3 wherein said resonator is held at a temperature below the superconducting transition temperature using a closed cycle refrigerator.
 6. A method as in claim 4 wherein said resonator is held at a temperature below the superconducting transition temperature using a closed cycle refrigerator.
 7. A method as in claim 1 wherein said resonator comprises dielectric material.
 8. A method as in claim 1 wherein a microwave source is provided for supplying said resonator with microwave radiation.
 9. A method as in claim 1 wherein an amplifier is provided, coupled to said resonator, to amplify power in said resonator.
 10. Apparatus for accurately determining a response characteristic of a microwave device, said apparatus comprising:a microwave resonator adapted for connection to said device; a switch connected to cause controlled cessation of energy input to said resonator without disconnection of said device from the resonator; and a time decay measurement circuit connected to monitor the decay of microwave energy passing from the resonator to the device after said cessation of energy input occurs.
 11. A method for accurately determining a response characteristic of a microwave device, said method comprising:connecting a microwave resonator to said device; energizing and de-energizing said resonator by a switch connected controlled cessation of energy input to said resonator without disconnection of said device from the resonator; and monitoring the decay of microwave energy passing from the resonator to the device after said cessation of energy input occurs.
 12. Apparatus for determining a response characteristic of a microwave device, said apparatus comprising:a microwave resonator; means for coupling said microwave resonator to said device; means for supplying said resonator with microwave electromagnetic energy until a steady state condition is reached in which input power equals the power dissipated in and radiated from said resonator; cutting off the supply of microwave electromagnetic energy to said resonator; means for monitoring the time decay of power from said resonator; and means for determining the response characteristic of said device in dependence on the monitored time decay.
 13. Apparatus as in claim 12 wherein said determining means comprises means for comparing the monitored time decay with a known characteristic of the time decay of radiation in said microwave resonator when supplied with radiation.
 14. Apparatus as in claim 12 wherein said resonator comprises superconducting material and the apparatus comprises means for maintaining said microwave resonator at a temperature below its superconducting transition temperature.
 15. Apparatus as in claim 13 wherein said resonator comprises superconducting material and the apparatus comprises means for maintaining said microwave resonator at a temperature below its superconducting transition temperature.
 16. Apparatus as in claim 14 wherein the superconducting material has a transition temperature above 60K.
 17. Apparatus as in claim 15 wherein the superconducting material has a transition temperature above 60K.
 18. Apparatus as in claim 14 further comprising a closed cycle refrigerator for holding said microwave resonator at a temperature below its superconducting transition temperature.
 19. Apparatus as in claim 15 further comprising a closed cycle refrigerator for holding said microwave resonator at a temperature below its superconducting transition temperature.
 20. Apparatus as in claim 12 wherein said microwave resonator comprises dielectric material.
 21. Apparatus as in claim 12 including a microwave source for supplying said microwave resonator with microwave radiation.
 22. Apparatus as in claim 12 including an amplifier, coupled to said resonator, for amplifying power in said resonator.
 23. A method for accurately determining a response characteristic of a microwave device, said method comprising:(a) generating and supplying microwave energy from a microwave energy source to said microwave device; (b) cutting off the further generation of said microwave energy; and (c) thereafter monitoring the time rate of decay for continued flow of previously stored microwave energy from said source to said microwave device to determine a response characteristic of the microwave device; wherein the accuracy with which the stored energy decays without being connected to said device provides a reference time scale against which the determined characteristic may be accurately calibrated.
 24. A method as in claim 23 wherein said microwave energy source includes a super-conducting microwave resonator.
 25. A method as in claim 24 wherein said resonator is fed microwave energy from an external open-loop source.
 26. A method as in claim 24 wherein said resonator is connected to form part of a closed loop microwave oscillator.
 27. A method as in claim 24 wherein said resonator stays constantly connected in circuit with said device during all of steps (a), (b) and (c).
 28. Apparatus for accurately determining a response characteristic of a microwave device, said apparatus comprising:(a) means for generating and supplying microwave energy from a microwave energy source to said microwave device; (b) means for cutting off the further generation of said microwave energy; and (c) means for thereafter monitoring the time rate of decay for continued flow of previously stored microwave energy from said source to said microwave device to determine a response characteristic of the microwave device; wherein the accuracy with which the stored energy decays without being connected to said device provides a reference time scale against which the determined characteristic may be accurately calibrated.
 29. Apparatus as in claim 28 wherein said microwave energy source includes a superconducting microwave resonator.
 30. Apparatus as in claim 29 wherein said resonator is in open loop connection to an external microwave source.
 31. Apparatus as in claim 29 wherein said resonator is connected to form part of a closed loop microwave oscillator.
 32. Apparatus as in claim 29 wherein said resonator is constantly connected in circuit with said device while the time rate of decay is being monitored. 